1. Field of the Invention
The present invention relates to an ultrasonic motor and a method for operating the ultrasonic motor.
2. Description of Related Art
One example of a previously proposed ultrasonic motor is a standing-wave type ultrasonic motor, such as shown in FIGS. 16 and 17. This type of ultrasonic motor includes a rotor 101 and a stator 102. The stator 102 includes metal blocks 104, 105, piezoelectric elements 106, 107, drive electrode plate 108, longitudinal-vibration sensing electrode plate 109 and common electrode plates 110, 111, which are all connected and fastened together by a single bolt 112. With reference to FIG. 16, the drive electrode plate 108 and the longitudinal-vibration sensing electrode plate 109 are integrated in a single disk and are electrically insulated from each other.
With reference to FIG. 17, a portion of the bolt 112 that protrudes from a top surface of the stator 102 (metal block 104) is received within the rotor 101, and a nut 113 is tightened onto the bolt 112 to tightly connect the rotor 101 and the stator 102 together.
The ultrasonic motor is rotated both forward and backward by a drive control circuit 119. With reference to FIG. 17, the drive control circuit 119 includes a rotational direction selection circuit 120, a frequency variable oscillation circuit 121 and a power amplifier 122. The rotational direction selection circuit 120 outputs a forward rotational signal s1 and a backward rotational signal s2 to the frequency variable oscillation circuit 121 when the ultrasonic motor is rotated forward and backward, respectively. The frequency variable oscillation circuit 121 generates a signal SGf1 having a resonance frequency f1 (or a signal SGf2 having a resonance frequency f2) for rotating the ultrasonic motor forward (or backward) based on the forward rotational signal s1 (or backward rotational signal s2) outputted from the rotational direction selection circuit 120 and then outputs it to the power amplifier 122. The power amplifier 122 amplifies the signal SGf1 having the frequency f1 (or the signal SGf2 having the frequency f2) and applies it between the drive electrode plate 108 and each one of the common electrode plates 110, 111.
Then, the ultrasonic motor is rotated forward with the high frequency voltage that has the resonance frequency f1 and that has been amplified through the power amplifier 122. Vibrations of the stator 102 generated during the forward rotation of the ultrasonic motor are complex vibrations that include torsional vibrations (mainly secondary torsional vibrations) as a major component and additionally include longitudinal vibrations as a minor component. Also, the ultrasonic motor is rotated backward with the high frequency voltage that has the resonance frequency f2 and that has been amplified through the power amplifier 122. Vibrations of the stator 102 generated during the backward rotation of the ultrasonic motor are complex vibrations that include longitudinal vibrations (mainly primary longitudinal vibrations) as a major component and additionally include torsional vibrations as a minor component.
A change in an ambient temperature or a load applied to the ultrasonic motor may cause the ultrasonic motor (stator 102) to vibrate at a frequency other than the resonance frequency f1 although the high frequency voltage having the resonance frequency f1 for the forward rotation is applied to the ultrasonic motor from the power amplifier 122. This results in reduced rotational efficiency of the ultrasonic motor. The same thing happens when the ultrasonic motor is rotated backward upon application of the high frequency voltage having the resonance frequency f2 for the backward rotation to the ultrasonic motor.
In order to vibrate the ultrasonic motor at the resonance frequency f1 (or resonance frequency f2) regardless of the change in the ambient temperature or the load, the frequency of the high frequency voltage to be applied between the drive electrode plate 108 and each one of the common electrode plates 110, 111 is controlled. More specifically, the drive control circuit 119 includes a vibration comparator circuit 130 and a frequency control circuit 131.
The vibration comparator circuit 130 receives a signal indicative of a current vibrational state of the stator 102 from a longitudinal-vibration sensing electrode plate 109 and thereby obtains a vibrational frequency (actual vibrational frequency) of the stator 102. The vibration comparator circuit 130 compares the actual vibrational frequency with the resonance frequency f1 (or the resonance frequency f2 in the case of the backward rotation) and outputs this comparison result to the frequency control circuit 131.
The frequency control circuit 131 computes a required control amount to shift the actual vibrational frequency of the stator 102 to the resonance frequency f1 (or the resonance frequency f2 in the case of the backward rotation) based on the comparison result and outputs the computed control amount to the frequency variable oscillation circuit 121. The frequency variable oscillation circuit 121 shifts the frequency of the signal SGf1 (or the signal SGf2) in such a manner that the actual vibrational frequency of the stator 102 substantially coincides with the resonance frequency f1 (or the resonance frequency f2 in the case of the backward rotation) based on the control amount outputted from the frequency control circuit 131 and outputs it to the power amplifier 122.
Thus, even though the ambient temperature or the load changes, the ultrasonic motor can vibrate at the resonance frequency f1 (or the resonance frequency f2 in the case of the backward rotation), so that the ultrasonic motor can be rotated effectively.
However, the vibrations of the stator 102 are complex vibrations comprising the longitudinal vibrations and the torsional vibrations. A vibrational pattern of the longitudinal vibrations is different from a vibrational pattern of the torsional vibrations. Thus, it is difficult to accurately sense the vibrational pattern of each one of the longitudinal and torsional vibrations with the single longitudinal-vibration sensing electrode plate 109. Furthermore, the previously proposed ultrasonic motor uses the vibration comparator circuit 130 and the frequency control circuit 131 which are rather complex and expensive, resulting in an increase in a manufacturing cost of the ultrasonic motor.
Thus, it is an objective of the present invention to provide an ultrasonic motor that can optimize its operating conditions and that allows a reduction in a manufacturing cost of the ultrasonic motor. It is another objective of the present invention to provide a method for operating such an ultrasonic motor.
To achieve the objectives of the present invention, there is provided an ultrasonic motor including a rotor and a stator. The stator generates complex vibrations for rotating the rotor. The complex vibrations include longitudinal vibrations and torsional vibrations. The rotor is rotatably urged against the stator. The stator includes a plurality of piezoelectric drive elements, a plurality of power supply electrode plates for supplying power to the piezoelectric drive elements, a longitudinal-vibration sensing means for sensing the longitudinal vibrations, a first metal block, a second metal block and a torsional-vibration sensing means for sensing the torsional vibrations. The piezoelectric drive elements, the power supply electrode plates and the longitudinal-vibration sensing means are securely clamped between the first metal block and the second metal block. The torsional-vibration sensing means is provided separately from the piezoelectric drive elements and the power supply electrode plates.
There is also provided a method for operating an ultrasonic motor including a stator and a rotor. The stator generates complex vibrations for rotating the rotor. The complex vibrations include longitudinal vibrations and torsional vibrations. The stator includes a plurality of piezoelectric drive elements, a plurality of power supply electrode plates for supplying power to the piezoelectric drive elements, a longitudinal-vibration sensing means for sensing the longitudinal vibrations, a first metal block and a second metal block. The piezoelectric drive elements, the power supply electrode plates and the longitudinal-vibration sensing means are securely clamped between the first metal block and the second metal block. The rotor is rotatably urged against the stator. The method includes steps of sensing voltage signals through the longitudinal-vibration sensing means and also through a torsional-vibration sensing means provided in the stator for sensing the torsional vibrations, generating a drive voltage signal for driving the stator based on the voltage signal in such a manner that an actual vibrational frequency of the stator substantially coincides with a resonance frequency of the stator, and applying the drive voltage signal to the power supply electrode plates.